To better understand the antigenic nature and the immune response to P. carinii, we have undertaken to characterize the major surface antigen of both rat and human P. carinii. It is necessary to use P. carinii from both sources because antigenically they are different and specifically the major surface antigen in rat and human are clearly different. We have previously purified the major surface glycoprotein in both rat and human pneumocystis using HPLC. Over the past year, we have identified a number of clones form a cDNA library of P. carinii that contain genes encoding for the major surface glycoprotein. The identity of these clones has been confirmed by obtaining amino acid sequence information from purified gp116. We have partial sequences on at least 7 clones and have a complete sequence on one of these clones. These clones are clearly related but not identical, very strongly suggesting that multiple genes encode the major surface glycoprotein. By pulse-field gel electrophoresis, these clones hybridize to all chromosomes. The predicted protein is high in cysteine and virtually all cysteine residues are conserved in all the clones. The predicted protein has a molecular weight of about 123,000. We have also generated peptides to poorly conserved regions for four of the clones that we have sequenced and have developed antibodies to these clones. In immunofluorescent and immunoblot studies we were able to show that two of these clones react preferentially with intact P. carinii. The goal of this study is to better understand the pathogenesis of P. carinii pneumonia with the hope that we can use this information to control of prevent this disease.